Wednesday, March 19, 2014

Oxidative Explorations of Taxanes

Our paper is now out. This
ongoing project, although some
might label as simple, requires many different types of oxidations, and all
of which require many experiments to discover. You can read the paper and ask
me questions or send me comments to your heart’s content, but in this post I
will talk about the discovery process and tell some of the stories.

Here is a graphical summary of the paper in one hilariously
hideous scheme (No, Phil, I don’t think it’s hideous
just because the fonts are in Arial):

We found the first oxidation (at C-5 of taxadiene (1)) in palladium-catalyzed
acetoxylation (treat your olefin with Pd(OAc)2 and benzoquinone in
acetic acid). This is one of those reactions that looks very clean by TLC and
crude NMR, but you only get 35% yield in the end. I wanted to see if I could
figure out where the starting material was going, so I ran the reaction in an
NMR tube with deuterated acetic acid and 1,3,5-trimethoxybenzene (TMB) as an
internal standard as the only deviations from the original experiment. I came
up with these data below:

I still couldn’t see any taxanes other than the starting
material and product, which was kind of frustrating, but I did notice this
reaction gave 42% yield with some SM left rather than 35% with no SM left.
After corroborating this result with the isolated yield of a separate larger-scale
reaction, we concluded TMB was somehow giving us the increased yield. A quick
screen of similar molecules led us to find that using only 4 equivalents of
anisole gives us a 49% yield. Like we describe briefly in the paper, we qualitatively observe
much less palladium black forming in these reactions containing anisole, so
maybe anisole facilitates reoxidation of Pd(0). This is a somewhat simplistic explanation because we saw 35% yield even when using stoichiometric Pd(OAc)2,
so maybe palladium black actually hinders the continued reaction. People
smarter than me have suggested anisole improves the yield because it is a “redox
buffer” or a “cation sponge,” but I don’t have any evidence for those ideas.

The next oxidation at C-13 was pretty extensively covered in
the paper, so I will be somewhat quick about it. We were getting byproducts 7 and 8 from Cr(VI) reagents like PCC and CrO­3∙3,5-dimethylpyrazole
complex (see Scheme 2 in the paper for a clearer view of this stuff, but you
can see it above too). I was in my first year as a grad student, and was trying
as many different Cr(VI) reagents we could think of, both known and unknown
(you can see a bit of my interesting screen near the bottom of the supporting
information). While I was putting my head down and working hard to find a
solution, Abraham came to me and said he was going to try this Cr(V) reagent (9) he found in the literature.
Apparently it was shown to be less effective at oxidative cleavage of
1,2-diols than Cr(VI) reagents. As the naïve student lacking the courage it takes to try something unknown
that I was, I told him he was crazy. This Cr(V) reagent gave us an improved
yield and a totally different byproduct. For a long time we couldn’t figure out
what to do with this reagent to determine its capabilities, but after thinking
about the mechanism for the byproduct’s formation we decided to see if it does
the allylic transposition of the Babler-Dauben reaction. It doesn’t (see bottom
rectangle in the above messy scheme), which we think is pretty interesting. Oh and I should note that we saw slightly lower yields with acetonitrile instead of trifluorotoluene as solvent, but with acetonitrile we didn't need to use 15-crown-5. So, for simplicity's sake, if you want to try this reagent on something new I would suggest using acetonitrile and no crown ether.

The third oxidation, perhaps unsurprisingly, comes with a
story too. Abraham and I tried quite a few types of oxidation with C-13 at
either the ketone or the alcohol oxidation state (corresponding to compounds 5 and 6 in the paper, respectively; see Figure 1 in the paper or the
horrible scheme above). We neglected one type of allylic oxidation, and it took
a brand new postdoc (Minetaka) to show us that simple radical halogenation—that
everyone learns in first-year undergraduate organic chemistry—was very
selective for C-10 oxidation. I find it super interesting that excess NBS brominates
C-18 (the neighboring vinyl methyl group) for enone 5, but treating diacetate 6
first brominates this C-18 methyl and then C-10. This experiment is actually
corroborated by computationally modelling these respective allylic radical
species, which is kind of cool too.

After discovering the first three allylic oxidations of our
synthetic oxidase phase, we devised a way to get to (–)-taxuyunnanine D (3). This took time because we wanted to
find a way to do it concisely because that’s sort of our thing. I think it
turned out pretty well.

What did I learn from these years of oxidative explorations?
Pay attention and maybe you’ll find something you weren’t expecting. Also, be
brave and try new and crazy things! Finally, spreading out the work among the
team, or even just discussing your work with others, will often lead to solving
long-standing problems.

What’s next? I am an oxidative explorer at heart so we will
keep at it. Eventually, we will get to Taxol. :-) Notes:I think this goes without saying for everyone else in the lab too, but I am totally willing to post or send my original FID files if you think I doctored my spectra or if you need them for some reason. This is the Open Flask after all.The current version of the paper online has mistakes, most notably the missing and cutoff words in Figure 1. It will be fixed soon. Edit (20 March 2014): Figure 1 is legible now, but some arrows are very pixelated. Still working on it.

23 comments:

For your first oxidation at C5, I have seen reports where people use different ligands (sulfoxide, sulfide or diazafluorinone) to get ok yields for allylic acetoxylation. What do you think about those conditions for this step?

As for sulfoxide, I only tried DMSO. I didn't try any of the bissulfoxide ligands from White. Correct me if I'm wrong, but I thought White used those to select for the linear allylic oxidation, which we did not desire (and no we never saw the linear product; the structure seems "spring loaded" for the branched product). No I did not try any sulfides. Yes on the diazafluorinone, but with no better results. And thanks!

I suppose the idea behind the MnO2 additive is to improve the conversion of the hypothetical compound 14 to 5? Did you ever try Lewis acid additives? What I'm thinking of is this paper (and of course all the papers that Agapie has recently published on redox inactive bimetallic systems):http://pubs.acs.org/doi/abs/10.1021/ja109056x

Yes that is our theory, and yes we'd love to trap out the alcohol 14 if we could. I know Abraham tried TMSCl (and maybe Ac2O and others, I don't remember), but I know TMSCl shut down the reaction. This paper with scandium looks very interesting though. I need to test some of these metal lewis acids out. Thanks for sharing former hoodmate!

Maybe the actual role of anisole (or 1,3,5-trimehoxybenzene) is modifying Pd catalyst and making it more stable. I have seen something similar with Pd(TFA).2py /O2 baloon/sieves/NaHCO3 oxidative cyclizations, they work only in toluene and xylenes. (With benzene or any other solvent, Pd black crashes out and there is no conversion. I suspected benzoic acid or a palladacycle but did not have time to investigate.)

There is one more possibility: The produced hydroquinone makes a very stable black 1:1 charge-transfer complex with hydroquinone (called quinhydrone) which not only ties up one equivalent but can gobble up your product also. Perhaps thats where your missing product is going and adding very electron rich aromatics (like anisole) helps to keep things soluble longer, by competing with hydroquinone in charge transfer complex formation.

Either way, there is one superbly electron rich additive (also a a good cation scavenger), that might be worth trying in place of anisole: pentamethylbenzene. It is available from Aldrich and Oakwood (we use it for debenzylation of aryl benzyl ethers in TFA)

Contrary to what HR says, we are an open flask and love to share. ;-) Here's a link to my google drive. It's a chemdraw file. I drew estrone for you. As a disclaimer, all of the publications that come out of this lab use Helvetica rather than the Arial in this file (sshh! I use a windows machine. Don't tell anyone. ◔_◔).

Ha! Well oxidizing allylic alcohols is a lot less finicky than benzylic oxidation, so I doubt water has anything to do with the oxidation step (but I don't know for sure because we always saw great yields and didn't do any optimization). The TES deprotection step, however, might require water since they added 5 eq water in the original Org Lett paper (link below, and they didn't say why they added water). We didn't add any water, but I won't vouch for the dryness of our DMSO bottle. This desilylation doesn't seem to be simple acid catalysis (read their paper), and water might have something to do with these SET oxidations people propose.

Great work! The best part to me is table S6 in the supporting information. A heatmap or bar graph - type of display is really called for here, with some kind of quantitation of the different products and extent of conversion.

Thanks! The thing with target-oriented synthesis ( as opposed to methodology) is that quantitation takes lower priority than "Did it work?" Material ends up being annoyingly precious when you're a few steps down the line, so screening reactions are done on 1 mg or less. I could show you TLC plates and crude NMR spectra for all of those reactions and you would probably be convinced of my written conclusions, but I would be scientifically uncomfortable to assign quantitative values. Plus, that kind of quantitation for this specialized taxane substrate will never be useful or instructive for anybody.

if the chromium(V) reagent(Na salt) gives best yields in PhCF3 (with 15-crown-5) but you prefer running the oxidation in MeCN so as to avoid using crown, maybe it would be worthwhile to combine the reagent in MeCN with Et4N(+)Cl, filter off NaCl and evaporate, then try to use the obtained tetraethylammonium salt for oxidation in low polarity solvent like halocarbons.

Et4N(+) salts are toxic, although not as nasty as Me4N(+) or high-valent chromium. (I would not recommend Bu4(+) because the chloride is expensive and the hydrogensulfate is acidic)

That is a great idea. I would encourage anyone trying this reagent to try this as well. We considered trying some different counter cations, but never got around to it.

And yes, adding toxicity to this reagent is the least of our worries. The vast majority of Cr(V) research in the literature is studying how it is probably the reactive intermediate in the process by which Cr(VI) causes DNA damage and, therefore, cancer. So don't go snorting this stuff or something.

Good work and nice paper BTW! Saw Baran talk about it recently at MIT.

Question:

Is CrV reagent "9" enantiopure? If not, is there a "matched/mismatched" chemo-diveregence when interacting with the (+)-taxol molecule thus leading to the different products? Also, the reagent "9" itself can be one of two diastereomers.

Thanks! No it is not enantiopure. We have thought about the ramifications of Cr(V) having enantiotopic alkyl groups on the ligands, but didn't come up with much because it does allylic oxidation to ketones and not alcohols. It might be interesting if we figure out a way to trap out the resulting alcohol. I suppose we might be able to get desymmetrization of a symmetrical diene, but that seems like a very specific application and there are probably better ways to do it. Yes, reagent 9 could be a mixture of diastereomers, but we saw essentially the same yields of product and byproduct when we used the methyl-methyl version (as opposed to 9, which we call the methyl-ethyl version), so my working hypothesis is that the stereocenters are too far away from the reagent's reactive site, which lends further credence to an outer-sphere SET mechanism. You might say "but there's not much difference between Me and Et," but we did try the Me-tBu version and got the same results too.

I'd first of all like to say congratulations on a great post - and really nice research too.

Just because you mentioned about making spectra available I wondered if you (and anyone else reading) would consider putting your spectra in ChemSpider as JCAMP files. Not because I doubt them, but so that they are available to anyone who needs them - and perhaps more importantly to give you more visibility. As an example I point to these spectra on the record for paracetamol (I know that they are not as exciting as yours!) http://www.chemspider.com/Chemical-Structure.1906.html#spectraA key aspect is being able to add comments and links so you can give context to the data and point people to your research group pages or to read the full paper.

As a declaration of interests - I do work on ChemSpider. If anyone has any interest or questions please do get in touch at chemspider -at- rsc dot org